42 THE NATURE AND WOBK OF PLANTS 



47. The tip of the root is protected hij a sheath- 

 ing cap. — The delicate tip of the root is pushed 

 through the soil very rapidly, and its apex must be 

 protected or it would be torn by the rough edges of 

 the rock particles. This may be readily realized 

 when it is found that the pressure by which the 

 root is driven forward is equal to fifteen or twenty 

 atmospheres, or two or three hundred pounds to the 

 square inch, much greater than that exerted by steam 

 in a locomotive boiler. 



If the root meets a soft or yielding substance, it 

 bores through it precisely as you might push a 

 large needle through the same mass. The young 

 roots of rapidly growing plants are often seen to 

 penetrate soft or decaying wood, or even the large 

 roots of other plants. The actual tip of the root 

 proper is composed of extremely delicate cells, with 

 the thinnest coverings in the way of cell walls, and 

 they would be crushed by the lightest touch of any 

 hard object. These cells are of importance because 

 by their division the tissues of the new portions of 

 the root are formed. To protect the delicate mass 

 of living matter most roots are furnished with a 

 sheath or cap on the tip. This root-cap has come 

 to serve other purposes as well, and it may be 



